The invention concerns a process for the preparation of organic mono- and polyisocyanates through thermal decomposition of trisubstituted ureas. The urea cleavage method, one of the most important phosgene-free isocyanate syntheses, is already known in principle; Ullmans Encyklopadie der Technischen Chemie, Verlag Urban & Schwarzenberg, Munich, Band IX, p.4/5 (3rd ed. 1957); Kirk-Othmer, Encyclopedia of Chemical Technology, John Wileys & Sons, Inc., Vol. 12, p. 54 (2nd ed. 1967); Houben-Weyl, Methoden der Organischen Chemie, Georg Thieme Verlag Stuttgart, p. 126 (4th ed. 1952). According to the prior art methods, the reaction is customarily effected in the vapor phase. The thermal decomposition of the trisubstituted urea is carried out at temperatures between about 135.degree. and 500.degree. C., in general in the presence of hydrogen chloride, phosphorus pentoxide or phosgene. The isocyanate is distilled off, leaving the secondary amine by-product as residue. Therein lies a major disadvantage of the vapor phase method: at the comparatively high temperature employed, substantial amounts of decomposition and polymerization products are formed.
Another known method involves the thermal decomposition of trisubstituted ureas in a solvent. According to the method of French Pat. No. 14 73 821, the trisubstituted ureas are decomposed at temperatures below 200.degree. C., generally in the presence of known catalysts such as tertiary amines or acetic acid, in a solvent with a dielectric constant at 20.degree. C. of less than 40, and preferably less than 10. This process nonetheless is low-yield and requires long reaction times of up to 35 hours.
In German DT-OS 22 25 365 a process is described, in which the thermal decomposition of trisubstituted ureas is effected in an inert organic solvent at temperatures between 100.degree. and 200.degree. C. in the presence of an excess of hydrogen chloride. A disadvantage of this method is the large amounts of hydrogen chloride and inert gas which must be introduced into the reaction solution. For this reason, the process is also not suited for the production of acid-sensitive isocyanates, which polymerize readily under the influence of the excess hydrogen chloride and thereby markedly reduce the yield.
Yet another method is described in U.S. Pat. No. 3,936,484, wherein the thermal decomposition takes place in the absence of a catalyst. With this method, however, comparatively high temperatures of between 230.degree. and 350.degree. C. are required and the reaction is carried out in an inert solvent. On account of the high reaction temperatures and the associated energy and technical requirements, this process must also be considered as disadvantageous.